The present invention relates generally to a thyristor controlled series capacitor in series with a substantially inductive transmission line, and more particularly to a firing control system, including a method and an apparatus, for secure vernier operation of a thyristor controlled series capacitor system, such as may be used in an alternating current (AC) power distribution system for damping subsynchronous oscillations and direct current (DC) offset.
Other systems have been proposed for damping subsynchronous oscillations and DC offset in power transmission systems. For example, U.S. Pat. No. 4,292,545 to Hingorani discloses a thyristor controlled capacitor in series with a transmission line. Hingorani's system uses a thyristor valve firing at a fixed time after a zero-crossing of the capacitor voltage. In U.S. Pat. No. 4,434,376 to Hingorani, which is a divisional of the Hingorani '545 patent, this concept is extended to have firing at a variable time after a capacitor voltage zero-crossing. In the Hingorani '376 patent, the firing time is adjusted relatively slowly, based upon a measurement of the frequency of the AC power system network voltage or current. In U.S. Pat. No. 4,355,241 also to Hingorani, overload protection is added by also triggering the thyristor valve in response to an over-voltage across a series reactance. This series reactance comprises a plurality of tuned circuits, each of which may be tuned to a separate undesirable subsynchronous frequency of the power system so the normal system frequency is unimpeded.
One serious drawback to the systems proposed by Hingorani in his '545, '376, and '241 patents (hereinafter and collectively referred to as "Hingorani") is that Hingorani requires several firings of the thyristor valves before the series capacitor voltage finally creeps-up to the desired level. Furthermore, after reaching the desired level, the Hingorani system typically overshoots the desired value. Other disadvantages of the Hingorani system are discussed further below.
In a 1988 paper by J.J. Vithayathil, et al. entitled, "Case Studies of Conventional and Novel Methods of Reactive Power Control on an AC Transmission System," it was suggested that by using partial conduction of an inductor across a series capacitor, the effective compensation level of the series capacitor could be varied substantially. Vithayathil also suggested that this system could be beneficial for stabilizing power systems. However, Vithayathil's inductor is controlled in the same way that a thyristor controlled reactor of a static VAR compensator is controlled, which is essentially the same as the Hingorani system. That is, both the Hingorani and Vithayathil systems are based on timing signals derived from the zero-crossing times of the capacitor voltage. The Vithayathil scheme also involves determining a firing time by assuming a purely sinusoidal voltage across the capacitor which, in actuality, is not a pure sinusoidal waveform.
In a 1990 paper by N. Christl, et al. entitled "Advanced Series Compensation With Variable Impedance," a "thyristor controller" mode is described which is similar to the "vernier" mode discussed further below. Christl shows several waveforms resulting from using the firing delay angle as the controlling variable.
Each of these earlier systems of Hingorani, Vithayathil, and Christl merely discloses a thyristor controlled, series capacitor system where the firing of the thyristor valves is based on a time determined from either the actual or the synthesized time of zero-crossings of the capacitor voltage. These systems are incapable of quickly and precisely damping subsynchronous oscillations and DC offsets in a power transmission system. Moreover, each of these firing systems is slow to respond.
None these earlier systems provides a secure means of controlling a thyristor controlled series capacitor where the actual capacitor voltage responds promptly to a command signal. Furthermore, these earlier systems are incapable of precisely controlling the duty on the thyristors. Additionally, these earlier systems are susceptible to drifting of the capacitor voltage away from a desired setting due to disturbances in the transmission line current.
Thus, a need exists for an improved firing control system for secure vernier operation of a thyristor controlled series capacitor used in power transmission systems to damp subsynchronous oscillations and DC offset, which is directed toward overcoming, and not susceptible to, the above limitations and disadvantages.